Fossil fuel is a general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years.
In common dialogue, fossil fuel, also known as mineral fuel, is used synonymously with other hydrocarbon-containing natural resources such as coal, oil and natural gas. The utilization of fossil fuels has enabled large-scale industrial development and largely supplanted water driven mills, as well as the combustion of wood or peat for heat. Fossil fuels are a finite, non-renewable resource.
When generating electricity, energy from the combustion of fossil fuels is often used to power a turbine. Older generators often used steam generated by the burning of the fuel to turn the turbine, but in newer power plants the gases produced by burning of the fuel turn a gas turbine directly. With global modernization in the 20th and 21st centuries, the thirst for energy from fossil fuels, especially gasoline derived from oil, is one of the causes of major regional and global conflicts.
The burning of fossil fuels by humans is the largest source of emissions of carbon dioxide, which is one of the greenhouse gases that allows radiative forcing and contributes to global warming. In the United States, more than 90% of greenhouse gas emissions come from the combustion of fossil fuels. In addition other air pollutants, such as nitrogen oxides, sulfur dioxide, VOCs, and heavy metals are produced.
Human activity raises levels of greenhouse gases primarily by releasing carbon dioxide from fossil fuel combustion, but other gases, e.g. methane, are not negligible. The concentrations of several greenhouse gases have increased over time due to human activities, such as burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations. According to the global warming hypothesis, greenhouse gases from industry and agriculture have played a major role in the recently observed global warming.